Advanced control strategies for chlorine dioxide generating processes

ABSTRACT

Chlorine dioxide generating processes of the single vessel type which produce chlorine dioxide of high purity are monitored and controlled by a computer using Advanced Control Strategies for steady, stable operation with optimum chemical usage on the basis of a desired chlorine dioxide production rate as the sole input from an operator to the computer program effecting the computer control.

FIELD OF THE INVENTION

[0001] The present invention relates to the generation of chlorinedioxide, particularly for pulp bleaching and, specifically, the controlof such process.

BACKGROUND TO THE INVENTION

[0002] Chlorine dioxide is produced commercially on site at a pulp millfor use as a bleaching agent for the pulp. A variety of procedures havebeen described in the prior art and have been used commercially. Ingeneral, the process proceeds in accordance with the equation:

ClO₃ ⁻+Cl⁻+2H⁺→ClO₂+½Cl₂+H₂O

[0003] One common procedure involves the so-called single vessel process(SVP), in which chlorine dioxide is formed from an aqueous acid reactionmedium, or generator liquor, which is maintained at its boiling point ina reaction zone under a subatmospheric pressure. Chlorine dioxide isremoved from the generator in gaseous admixture with water vapour and isabsorbed into chilled water in an absorption tower while by-product saltfrom the process precipitates in the generator and is removed therefrom.Make-up chlorate and acid are continuously added to maintain steadystate conditions in the generator liquor. A steam-fed reboiler in arecycle loop is used to maintain the generator liquor at its boilingpoint.

[0004] In this procedure, the chloride ion may be produced in situ byusing reducing agents which are believed to react with the chlorineco-produced in the process, such as methanol and hydrogen peroxide, inwhich case the chlorine dioxide is obtained substantially free fromchlorine. Alternatively, the chloride ion may be added as the reducingagent, in which case the chlorine dioxide is obtained in admixture witha significant proportion of chlorine.

[0005] Two key control variables in the production of chlorine dioxideare the acid normality and chlorate molarity in the generator liquor.Unfortunately, a viable on-line instrument to measure these key controlvariables is not commercially available and hence presently, an operatormust rely upon the results of manual laboratory tests, performed usuallyat two hours intervals, to guide him in making adjustments to themanipulated variables (i.e. acid and chlorate feed rates) to maintainchlorine dioxide production at the desired level. The control is furthercomplicated by fluctuations in generator liquor level that result in thegenerator liquor species either concentrating or diluting in theintervals between laboratory tests and thus it is difficult to controlthese variables using conventional control strategies.

[0006] Modern control systems found in most chlorine dioxide plants areequipped with microprocessors capable of rapidly computing complex,multivariable algorithms. This advancement in computing technology hasprovided the opportunity for the optimization of chemical processesthrough the implementation of advanced control strategies. It has longbeen desired to provide a series of advanced control strategies intendedto closely supervise the operation of the chlorine dioxide plant withthe sole input being the target chlorine dioxide production rate. Thisconcept is realized by the present invention as described in detailbelow. The entire plant operation can be manipulated by the advancedcontrol strategies, which may be implemented by a microprocessor.Laboratory tests are initially required to establish plant specificchemical consumption ratios, but once such initial testing is completed,the frequency of laboratory tests can be substantially reduced.

[0007] Previous methods for controlling chlorine dioxide processes areset forth in the following U.S. patents: U.S. Pat. No. Patentee4,251,503 Cowley et al. 4,251,224 Swindells et al.

[0008] Both cited patents, assigned to the assignee hereof, relate tothe machine control of chlorine dioxide generating processes, usingchloride ions as the reducing agent, producing a gaseous mixture ofchlorine and chlorine dioxide. Both patents disclose a method ofadjusting operating parameters as a finction of efficiency determinedfrom gas analysis (i.e. comparing the ratio of chlorine dioxide tochlorine). The present invention, however, addresses the advancedcontrol strategies with novel concepts to control modern,environmentally-friendly processes generating chlorine dioxide of highpurity (e.g. using methanol as the reducing agents, such as R8®,SVP-MeOH® and SVP-Lite®) with only trace amounts of chlorine present inthe product. The control strategies of the prior art and the invention,therefore, relate to the chlorine dioxide processes using differentchemistry.

SUMMARY OF THE INVENTION

[0009] As indicated above, the present invention is concerned withadvanced control strategies for controlling chlorine dioxide generatingprocesses of the single vessel type which produce chlorine dioxide ofhigh purity and the implementation of such control strategies using asuitably programmed microprocessor. The control strategies providedherein steer all key process variables on a dynamic basis andadjustments are made instantly.

[0010] The goals of this invention are listed below:

[0011] Target chlorine dioxide production rate achieved and maintained,

[0012] Steady, stable operation under optimum operating conditions,

[0013] Operation monitored and controlled by a computer,

[0014] Operator involvement and frequency of manual laboratory testingdecreased,

[0015] Plant performance optimized and chemical savings realized.

[0016] In one aspect of the present invention, there is provided acontinuous process for the generation of chlorine dioxide at apredetermined production rate, which comprises reducing chlorate ions,generally provided by sodium chlorate, chloric acid or mixtures in anaqueous acid reaction medium in a reaction zone using a reducing agentand sulfuric acid at the boiling point of the reaction medium under asubatmospheric pressure; removing a gaseous admixture comprising watervapour and chlorine dioxide from the reaction medium; absorbing saidgaseous admixture in chilled water in an absorption zone to provide aproduct aqueous solution of chlorine dioxide; removing a slurry of spentreaction medium and by-product crystalline sodium sulfate from thereaction zone; separating the crystalline sodium sulfate as a by-productfrom spent reaction medium; adding make-up quantities of sodiumchlorate, reducing agent and sulfuric acid to the spent reaction mediumto form a make-up feed; evaporating water inputted to the process fromall sources using steam fed to a reboiler; recycling the make-up feed tothe reaction zone; and computer controlling said process on the basis ofa desired chlorine dioxide production rate as the sole input from anoperator to a computer program effecting such computer control.

[0017] The computer control operation may comprise continuouslymonitoring the target production rate of aqueous chlorine dioxidesolution for changes therein, continuously monitoring the flow rates ofsodium chlorate, reducing agent, sulfuric acid, reboiler steam andchilled water to the process, and modifying the initial set points ofall said flows in accordance with the changed target production rate.

[0018] The computer control operation also may comprise continuouslymonitoring the production rate of aqueous chlorine dioxide solution fordeviations from the target production rate, and modifying the reducingagent flow rate to maintain the production rate at its target.

[0019] In one feature of the invention, the maximum allowable chlorinedioxide product solution strength and maximum allowable temperature aredetermined and advised to an operator.

[0020] The computer control operation may further comprise continuouslymonitoring the specification of all material feeds, and modifying theappropriate flow rate set points of the feeds to the reaction zone basedon the target production rate and in response to changes in materialspecification.

[0021] The computer control operation may additionally comprisecontinuously monitoring sodium chlorate solution physical properties,temperature and density, and, on this basis, creating an on-line virtualchlorate solution analyzer that determines the volumetric concentrationof the sodium chlorate solution.

[0022] The on-line virtual chlorate solution analyzer provides anaccuracy of about ±0.3% in the sodium chlorate concentration range ofabout 450 to about 750 gpL.

[0023] The computer control operation may further comprise continuouslymonitoring the mass input of sodium chlorate to the reaction medium,continuously monitoring the mass consumption of sodium chlorate by theprocess, and modifying the flow of sodium chlorate to the reactionmedium to correspond to the mass consumption of sodium chlorate so as tomaintain the sodium chlorate concentration in the reaction mediumsubstantially constant.

[0024] The computer control operation may further comprise establishingthe boiling temperature set point of the reaction medium based on theexpected reaction medium composition, continuously monitoring thetemperature of the aqueous acid reaction medium, continuouslycontrolling the temperature of the reaction medium in order to maintaina constant acid normality in the reaction medium, and continuouslydetermining the acid normality of the aqueous acid reaction medium fromthe temperature of the aqueous solution.

[0025] In the latter procedure, the computer control operation mayfurther comprise continuously determining whether the temperature of theaqueous reaction medium differs from the temperature set point, andcorrecting such deviation by suitable modification to the acid flow rateto the aqueous reaction medium.

[0026] The computer control operation may additionally comprisecontinuously controlling sodium chlorate molarity in the aqueousreaction medium on the basis of continuously determined system massbalance and adaptive yield tracking.

[0027] The computer control operation may further comprise periodicallylaboratory testing the concentration of sodium chlorate in the reactionmedium and monitoring the results of such laboratory testing for a trendin alteration of the concentration of the sodium chlorate in thereaction medium, determining whether or not the concentration of sodiumchlorate in the reaction medium has changed in the same direction in apredetermined number of said periodic laboratory tests, in the event,such a change has taken place and provided that the operator hasselected the “ADAPTIVE YIELD” function switch, initiating a yieldcalculation using a series of laboratory tests to determine theapplicable adaptive yield.

[0028] The computer control operation may additionally compriseperiodically laboratory testing the concentration of sodium chlorate inthe reaction medium, determining whether or not the concentration ofsodium chlorate in the reaction medium has changed from a target value,and in the event such a change has taken place and provided the operatorhas selected the “LAB TEST” function switch, applying a one-time bias tothe flow rate of sodium chlorate to the reaction medium for apredetermined time to adjust the sodium chlorate concentration in thereaction medium to the target value.

[0029] The computer control operation may further comprise maintainingthe level of reaction medium in the reaction zone substantially constantby continuously balancing the volume of water flowing to the process andthe volume of water evaporated from the reaction medium.

[0030] In the computer control operation, the acid normality of thereaction medium and the concentration of sodium chlorate in the reactionmedium may be continuously determined and displayed.

[0031] The reducing agent utilized in the chlorine dioxide generatingprocess may be of those commonly employed in commercial chlorine dioxidegenerating operations, preferably a reducing agent which does notproduce significant amounts of chlorine, such as hydrogen peroxide andmethanol. The specific description herein is directed to the use ofmethanol as the reducing agent.

[0032] Where methanol is the reducing agent, the computer control systemmay further comprises continuously monitoring the production rate ofaqueous chlorine dioxide solution, and modifying the feed rate ofmethanol to the reaction medium in response to fluctuations within apredetermined range based on the initial methanol flow set point.

[0033] The advanced control strategies illustrated in this invention(see FIG. 3 for schematic overview) include:

[0034] (a) Production Rate Initialization

[0035] (b) Dynamic Determination of Feed Rate Set Points

[0036] (c) Chlorate Feed Control

[0037] (d) Generator Liquor Acidity Control

[0038] (e) Production Rate Feedback Control

[0039] (f) Reboiler Steam Set Point Determination

[0040] (g) Generator Level Control

[0041] (h) Chlorine Dioxide Solution Strength Control

[0042] (i) Maximum Chlorine Dioxide Solution Strength and SolutionTemperature Interlock Setting

BRIEF DESCRIPTION OF DRAWINGS

[0043]FIG. 1 is a schematic diagram of a methanol-based chlorine dioxidegenerating plant (R8®) which can be controlled according to oneembodiment of the control strategies provided herein;

[0044]FIG. 2 is a schematic diagram of all inputs and outputs of thechlorine dioxide generating process control strategy provided herein;

[0045]FIG. 3 is an overall flow chart for the chlorine dioxidegenerating process control strategy provided herein;

[0046]FIG. 4 is a flow chart showing the steps involved in chlorinedioxide production rate initialization;

[0047]FIG. 5 is a flow chart showing the steps involved in the sulfuricacid set point determination;

[0048]FIG. 6 is a flow chart showing the steps involved in the methanoldilution water set point determination;

[0049]FIG. 7 is a flow chart showing the steps involved in the set pointdetermination for the flow of chilled water to the absorption tower;

[0050]FIGS. 8A, 8B and 8C represent a flow chart showing the stepsinvolved in the feed flow control of sodium chlorate solution;

[0051]FIG. 9 is a flow chart showing the steps involved in chlorinedioxide generator acidity control;

[0052]FIG. 10 is a flow chart showing the steps involved in chlorinedioxide production rate feedback control;

[0053]FIGS. 11A and 11B represent a flow chart showing the stepsinvolved in the reboiler steam flow set point determination;

[0054]FIGS. 12A and 12B represent a flow chart showing the stepsinvolved in chlorine dioxide generator liquor level control;

[0055]FIGS. 13A and 13B represent a flow chart showing the stepsinvolved in interlock setting for maximum chlorine dioxide solutionstrength and temperature; and

[0056]FIG. 14 is a schematic representation of the Remote AdvancedControl Architecture.

GENERAL DESCRIPTION OF THE INVENTION

[0057] In the present invention, the full scope of chlorine dioxidegenerating plant operation can be monitored and controlled automaticallyby a computer programmed by software effecting the various monitoringand calculations. The chlorine dioxide production rate is the sole inputvariable required to be provided by the plant operator to the system. Attimes, due to various reasons, the plant may not produce the amount ofchlorine dioxide expected from given flows of chemicals. The controlsystem provided herein rapidly determines the degree of production ratedeviation and makes the required adjustments to the methanol feed setpoint to maintain the desired production rate. Not only does the controlsystem optimize steady state chlorine dioxide generating plantoperations but also transitions between desired production rates, withcontinued stable operation. Stable generator level control at steadystate, for example, ±1%, is achieved by the synergistic use of steam andmake-up water. Water load and steam usage are minimized.

[0058] The present invention controls the chemical flows for optimalchemical usage at the target production rate. The chlorate concentrationin the generator liquor is maintained within a narrow variation range,for example, ±0.2M (analysis error included), at the target productionrate using adaptive yield compensation, as described below. The acidityof the generator liquor is controlled within a narrow variation range,for example, ±0.2N (analysis error included), at the target productionrate by maintaining the liquor temperature at its set point beingderived from the target generator liquor concentrations.

[0059] The chlorate feed concentration is calculated within goodaccuracy, for example, ±0.3% on the basis of a physical propertiesrelated algorithm, in the range of sodium chlorate concentration ofabout 450 to about 750 g/L.

[0060] The present invention enables there to be provided real timesodium chlorate and acidity predictors which can be displayed on thedistributed control system (DCS) at the plant for reference.Instrumentation errors can be detected by the present invention andcompensated for in most cases.

[0061] The enhanced accuracy in product aqueous chlorine dioxidesolution strength provided by the present invention minimizes theproduction of “off spec” chlorine dioxide solution during changes inplant production rate, with consequentially-improved pulp millbleaching. By monitoring the operating conditions of the chlorinedioxide generator, the recovery efficiency of the chlorine dioxideabsorption tower is optimized. The highest possible chlorine dioxidesolution strength that can be produced based on the advisory of maximumallowable chlorine dioxide solution strength under the existingoperating conditions cannot be exceeded, so that product losses,environmental release and safety incidences may be avoided. With theproduction of the highest possible chlorine dioxide solution strength,existing storage capacity is maximized and chilled water consumption isminimized. The chilled water flow to the absorption tower is preciselycalculated to maintain the chlorine dioxide solution strength at its setpoint following changes in production rate. By closely-controlling theoperating conditions at the target production rate, a maximum andsustained reaction efficiency is achieved, resulting in a higher yieldof aqueous chlorine dioxide solution based on the chlorate feed.

[0062] The present invention assists in reducing chemical costs,particularly chlorate; chemical losses due to production swings,disturbances at start-up; upsets, white-outs, liquor carry-over andenvironmental excursions; variation between target and operatingparameters, such as generator level, liquor concentrations, chlorinedioxide strength and production rate; frequency of lab tests; andbiological oxygen demand (BOD) loading through methanol consumptionreduction.

[0063] The present invention assists in improving bleached pulp productquality, as a result of consistent chlorine dioxide concentration in theproduct chlorine dioxide solution leading to improved pulp brightnesscontrol; stability of chlorine dioxide generating plant operation; andtroubleshooting ability.

[0064] The use of remote control of the chlorine dioxide generatingplant operation, as contemplated in one embodiment herein, providesadditional advantages over conventional local supervisory control,including remote software upgrades, maintenance and timely and enhancedtechnical service and support without the necessity for costly sitevisits, there being no risk in this mode of operation since anycommunication failure between central control centre and theremotely-controlled plant only results in controllers reverting to thelocal operator control as it is today.

[0065] The control strategies provided herein substantially enhance theoperational stability of the chlorine dioxide generation process, whichis the primary factor to affect the raw material usage. In addition, themaintenance of uniform operating conditions enables the process tomaintain the optimal generation of chlorine dioxide. The consumption ofthe most expensive material, sodium chlorate, is saved significantly bythe control system described herein.

[0066] Methanol consumption strongly relies on the generator liquorconcentrations. With operation of the control system provided herein,the generator liquor concentrations are held constant and at theoptimum, and as a result, methanol is utilized efficiently in thechlorine dioxide generating process. Therefore, there arises a chemicalsaving on methanol and a reduction on BOD load of the process.

[0067] In addition, the Advanced Control Strategies leads to saving onutilities, especially on the reboiler steam as a result of minimum useof make-up water in generator liquor level control. A chemical saving isrealized at the bleach plant utilizing the chlorine dioxide solution,that is benefited from high and consistent strength of chlorine dioxideproduct.

DESCRIPTION OF PREFERRED EMBODIMENT

[0068] Referring to the drawings, FIG. 1 is a schematic diagram of amethanol-based chlorine dioxide generating plant 10 which employssubatmospheric pressure and boiling reaction conditions to producechlorine dioxide from sodium chlorate, sulfuric acid and methanol.

[0069] As may be seen therein, the plant 10 includes a single vesselgenerator-evaporator-crystallizer 12 which has a recycle loop 14 whichincludes a reboiler 16. A gaseous product pipe 18 leads from thegenerator 12 through an indirect contact cooler 20 to an absorptiontower 22, to which chilled water is fed by line 24. Chlorine dioxidesolution is fed by line 26 from the absorption tower 22 to a chlorinedioxide solution storage tank 28.

[0070] A slurry of crystalline sodium sulfate by-product and spentgenerator liquor which enters the recycle loop 14 at the lower end ofthe generator 12 is pumped by line 30 to a salt cake filter 32 whereinthe by-product sodium sulfate is separated from spent generator liquor,which then is returned to the recycle loop 14 by line 34.

[0071] A feed line 36 for aqueous chlorate solution is provided upstreamof the reboiler 16 while feed lines 38 and 40, respectively for sulfuricacid and aqueous methanol solution, are provided downstream of thereboiler 16. Steam is fed to the reboiler 16 by line 42 to maintain theaqueous acid reaction medium in the generator 12 at the desired reactiontemperature and to evaporate all water inputs from all sources.

[0072] The aqueous acid reaction medium in the generator 12 ismaintained at its boiling point by applying a subatmospheric pressure tothe generator 12 by employing steam ejectors 44 connected to theabsorption tower 22 by line 46. The chlorine dioxide storage tank 28 isprovided with a vent line 52 to a vent scrubber 54 to which chilledwater is fed by line 56 to scrub chlorine dioxide from the vent gasstream.

[0073] In accordance with the present invention, the production ofchlorine dioxide by the operation of the chlorine dioxide generatingplant shown in FIG. 1 is controlled to provide a desired production rateof chlorine dioxide. The sole input parameter required by thepredetermined control system is the production rate. Based on thisparameter, the other parameters and operating condition of the processautomatically are varied to meet the target production rate.

[0074] The operating parameters of the chlorine dioxide generatingprocess are controlled by the control system, by receiving inputs ofinstantaneous values of various parameters of the process and adjustingflows and conditions as required to optimize the values which result ina predetermined production rate for chlorine dioxide. The productionrate of chlorine dioxide is the determining parameter with respect tooperation of the control system.

[0075] As may be seen in FIG. 2, production rate is an input value tothe control system 50, the operation of which is described below withreference to FIGS. 3 to 13.

[0076] Other inputs include chlorine dioxide absorber temperature,chlorine dioxide generator pressure, liquid level, temperature andchlorate concentration. As noted above, chilled water is fed to thechlorine dioxide absorber 22 while sodium chlorate, sulfuric acid,methanol and reboiler steam are fed to the chlorine dioxide generator.Each of these liquid feeds as well as make-up water (not shown) havefeed rates which are monitored and controlled by the control system 50.The density of the sodium chlorate solution and methanol also aremonitored by the control system 50.

[0077] The operating parameters for the chlorine dioxide generationprocess are selected from those normally encountered in a methanol basedchlorine dioxide generating process and maintained at the respectivevalues.

[0078] Turning now to FIGS. 3 to 13, FIG. 3 shows the overall controlscheme. The overall control scheme is contained within a computerprogram run by a suitable microprocessor. As mentioned above, targetproduction rate is the input determinative of the operation of thecontrol system. This input is introduced manually by an operator. Adeviation in production rate from the target causes the system to makesuitable adjustments to return the process to the existing productionrate target.

[0079] As seen in FIG. 4, the target chlorine dioxide production rate isinputted from the DCS and the control system detects whether there isany change in production rate. In the event that there is a change inproduction rate, a signal initiates START 2 to initiate changes in feedset points, as described below.

[0080] The conventional operation of a chlorine dioxide generation plantdoes not involve chlorine dioxide production rate initialization, unlikethe present invention, but rather all chemical feed set points aremanually inputted in the conventional system to achieve a targetproduction rate. This invention uses the target production rate as atrigger to initialize subordinate control strategies in a logicalsequence in order to achieve smooth transitions from one target chlorinedioxide production rate to the next.

[0081] The control strategies provided herein have shown a superiorability to handle transitions, for instance, at the time of productionrate change. A large jump in production rate was experimented with andthe chlorine dioxide strength, which generally is the first parametersensitive to process swings, was maintained close to the set point whilepassing the transition period to the new production rate.

[0082] START 2 initiates acid flow, methanol dilution water and chilledwater set point determinations, which provide new set pointdetermination responsive to the change in target chlorine dioxideproduction. As seen in FIG. 5, the control system provides informationrelating to the existing sulfuric acid consumption, sulfuric acidconcentration and target chlorine dioxide production rate and then thenew sulfuric acid flow set point (SP20) is calculated, which isforwarded to the DCS to effect the necessary adjustment in sulfuric acidflow and a signal initiates START 4.

[0083] As seen in FIG. 6, the DCS provides methanol consumption,methanol solution density and target chlorine dioxide production rateand then the control system calculates the new methanol flow set pointand the new methanol dilution water flow set point (SP30) which is fedto the control system to effect the necessary adjustment and a signalinitiates START 5.

[0084] As seen in FIG. 7, the DCS provides chlorine dioxide solutionstrength set point, reboiler steam flow, chilled water flow to ventscrubber and target chlorine dioxide production rate, then the controlsystem calculates the total required absorption water flow at the targetproduction rate, calculates the condensate flow from the indirectcontact cooler to the absorption tower, calculates the new chilled waterto absorption tower flow set point (SP70). The minimum chilled water toabsorption tower flow (SPMN) is calculated and a comparison is made todetermine if SP70 is greater than SPMN in order to ensure that thechilled water flow would never drop below the minimum SPMN. If theanswer is no, then the SP70 value is set as the calculated SPMN valueand then this value is fed to the DCS to make the adjustment. If theanswer is yes, then the SP70 value is set as the calculated SP70 valueand then this value is fed to the DCS to make the adjustment.

[0085] In addition, a further query is made as to whether the chlorinedioxide strength or the scrubber flow set points have changed. In theevent the answer is no, then a signal initiates START 6. If the answeris yes, then the loop returns and adopts the new values of ClO₂ solutionstrength and/or vent scrubber tower flow for re-calculation.

[0086] In contrast to the control strategies outlined in FIGS. 5 to 7,the conventional operation employs a “Hardcopy Feed Table” as aguideline for operators to set all flows for the target production rate.This table is created on the basis of stoichiometry and a number ofassumptions, including constant methanol consumption and density,chlorate strength, and plant yield, which may be incorrect. Thisinvention makes the feed-forward set points dynamic, whereby the flowset points of acid, methanol, steam and chilled water are calculatedbased on a given chlorine dioxide production target, specifications ofchemical supply and directly measured operating conditions and the setpoints are changed dynamically as the material specifications vary. Thisstrategy allows the initial and ongoing flow set points to be as preciseas possible, unlike the conventional rigid feed chart.

[0087] In FIGS. 8A, 8B and 8C, there is shown the procedure for sodiumchlorate solution feed control. As seen in FIG. 3, controlinitialization assigns the first chlorate flow set point (SP10) and asignal initiates START 3. Information relating to sodium chloratesolution flow, density, temperature, chlorine dioxide flow to storageand chlorine dioxide solution strength is provided from the DCS and thencalculates average production rate, which generates a signal (7) for thechlorine dioxide production rate feedback control (see FIG. 10). Thesodium chlorate usage at the actual chlorine dioxide production rate andyield is then calculated, followed by calculation of the chlorateconcentration in the feed solution. This latter calculation generates asignal (8) for the reboiler steam flow set point determination (seeFIGS. 11A, 11B). The required chlorate flow rate to match the chlorateconsumption then is calculated. This value along with inputs 6A or 6C(see below) then are used to calculate the chlorate flow set pointincorporating bias and yield adjustment, as described below, and theresulting signal is forwarded to the DCS. The signal also initiatesSTART 7, is looped internally (5) and is used in further control of thechlorate feed flow (7).

[0088] As seen from the above description of FIGS. 8A to 8C, theoperator has the option to either rely on full complete control usingthe “ADAPTIVE YIELD” function switch, (FIGS. 8A and 8C), or to selectthe “LAB TEST” function switch for a one-time flow bias adjustment(FIGS. 8A and 8B).

[0089] As seen in FIG. 8B, the chlorate flow signal 5 is inputted alongwith information relating to chlorate morality and percent solidstargets, generator level and lab test results for chlorate molarity andpercent solids. Subsequently, it is determined whether the chlorate labtest information has changed. In the event, the answer is no, a closedloop is established. In the event, the answer is yes, then it isdetermined whether the adaptive yield mode, as described below, isselected. If the adaptive yield is selected, then a signal (6B) isgenerated from which further calculation are made (see FIG. 8C). If theadaptive yield is not selected (i.e. lab test mode is selected), thenthere are sequentially calculated the generator liquid volume at actualoperating conditions, the chlorate inventory is calculated at actualoperating conditions, the reference liquid volume at referenceconditions, the reference chlorate inventory at reference conditions andthe chlorate inventory deviation between the actual and referenceconditions. The chlorate flow adjustment bias then is calculated and thesignal (6A) applied for a predetermined time to the calculation of thechlorate flow set point (FIG. 8A).

[0090] If the adaptive yield is selected, then a comparison is made asto whether or not the number of lab tests performed has met the basiccriteria. If not, then the further calculations are by-passed untilthere have been a sufficient number of lab tests performed to meet thecriteria. If yes, then there are sequentially calculated the chlorinedioxide totalized mass output beginning from the first valid lab test,the chlorate totalized mass input beginning from the first valid labtest, the corrected chlorate consumption using the last valid lab testin comparison to the predicted chlorate concentration in the liquor andthe chlorate adaptive yield correction, the signal of which is used tocalculate the chlorate flow set point (FIG. 8A).

[0091] In contrast to the aqueous sodium chlorate feed control strategyillustrated in FIGS. 8A to 8C, the conventional operation in a chlorinedioxide generator is to make manual adjustments to the chlorate feedflow based on results of manual chlorate molarity laboratory tests ongenerator liquor. Adjustments are made assuming constant chlorinedioxide production, generator volume, yield and chlorate feed strength,which may be incorrect.

[0092] In the present invention, an on-line virtual chlorate solutionanalyzer is provided that reports the real volumetric concentration ofchlorate solution being used in the process based on solution physicalproperties, temperature and density. Taking advantage of the on-linechlorate solution analyzer, it becomes feasible to monitor the massinput of chlorate to match the consumption that is known from thechlorine dioxide production as well as predetermined chlorate yield,such that the chlorate inventory in the generator liquor indicated bythe chlorate molarity is constant, which leads to operational stability.

[0093] This invention has developed an empirical relationship ofchlorate solution density-temperature-concentration correlation based onconventional standard relationships. The empirical relationship isproven to be accurate within 0.3% relative error in the aqueous chloratesolution concentration range of about 450 to about 750 gpl. The on-linechlorate solution concentration signal is used in the continuous massbalance control. This control strategy makes continual adjustments onchlorate flow rate applying the averaged chlorine dioxide productionrate as well as the chlorate concentration determined by therelationship. The chlorate molarity in the generator liquor thus iscontrolled by system mass balance and adaptive yield tracking. Moreover,automatic bias adjustments may be made to chlorate feed flow after eachlaboratory test result is entered as described above with respect toFIGS. 8A and 8B.

[0094] Alternatively, the chlorate feed control is governed by adaptiveyield, whereby the chlorate yield, namely the molar ratio of chlorinedioxide produced to actual chlorate used, is derived and adjustedperiodically based on changes in the inventory of sodium chlorate in thegenerator liquor calculated from a series of lab tests.

[0095] As described above with respect to FIGS. 8A and 8C, in theadaptive yield mode mentioned above, the lab test data of chloratemolarity are saved and monitored for their trend. In the event thechlorate concentration value in the generator liquor moves three timesin a row in the same direction and does not pass the target, a yieldcalculation is initiated using the lab tests to determine the chloratemass inventory changes. The inventory changes then are used to adjustthe adaptive chlorate yield. As the chlorate mass inventory in thechlorine dioxide generator liquor rises, the yield is to be loweredwhile decreasing chlorate mass inventory in the chlorine dioxidegenerator liquor suggests a higher yield. By performing suchdetermination, the lab test data are not only used to create a one-timeinput bias to the chlorate feed, but also to direct the predeterminedyield to the true value thus ensuring the chlorate input is in thebalance with the consumption to produce chlorine dioxide.

[0096] The concept of adaptive yield employed herein recognizes that theplant instrumentation may contain some errors and the system factorsaffecting yield, such as instrumentation errors and the processcharacteristics, reaction efficiency and physical losses, may vary fromtime to time. With adaptive yield, the system is able to maintain itsstability in the ever-changing environment by following the stepsdescribed above.

[0097] The concept of adaptive yield employs trend analysis rather thanusing a single lab test, so that the effect of errors associated withthe sample analysis can be minimized. For instance, one point ofoff-trend line test can be ignored and the disturbance to the chlorinedioxide generating process that such test could have caused is averted.

[0098] A generator chlorate molarity predicator may be shown on the DCSat the chlorine dioxide generator plant, based on a comparison of massinput and output, as described above. The predictor serves to indicateto the plant operator the transition of chlorate molarity from one stateto a new balance.

[0099]FIG. 9 shows the steps taken in the generator liquor aciditycontrol. Information relating to sulfuric acid flow, generator liquortemperature, generator liquor target normality and molarity andgenerator pressure is provided from the DCS and the acid flow set pointlimits are calculated from the acid set point signal 1 (SP20) and theresulting signal is fed to the DCS. The generator liquor temperaturecontroller set point (T20) next is calculated and also sent to the DCS.The deviation of generator liquor temperature from the set point iscalculated and a determination is made if the deviation is greater thana permitted value (A′). If the answer is no, then the signal loops tothe start of the calculation. If the answer is yes, then a determinationis made if the deviation is less than the permitted value (A″). If theanswer to the latter determination is no, then the signal loops back tothe start of the calculation. If the answer to the latter determinationis yes, then along with the calculated new acid flow set point (input 1,see FIG. 5), the current acid flow set point is reset to the previouslycalculated acid flow set point that is forwarded to the DCS and loopedinternally. Due to long time delay in response, it was experienced thata simple acid flow-liquor temperature control loop would encounter longlasting swings, especially when upsets occurred. A one-time acid flowcorrection, as effected herein, enables the liquor temperature to returnrapidly to its set point and swings cease. This latter signal triggersSTART 7.

[0100] The conventional procedure for generator acidity control is tomake manual adjustments to the acid feed flow based on results of acidnormality laboratory tests on generator liquor. Adjustments are madeassuming constant chlorine dioxide production, generator volume and acidconsumption which may be incorrect. This invention does not require theacidity laboratory test but instead manipulates the acid feed flow as afunction of generator liquor temperature (i.e. boiling point) alone,with the initial acid flow set point given by the mass balancecalculation such that the starting set point is as close as possible tothe real acid requirement.

[0101] This invention has developed an empirical relationship based onknown considerations of boiling point to liquor composition to determinethe desired generator liquor temperature control set point. Thecomputer-assigned temperature control set point is derived from expectedliquor composition, i.e. chlorate molarity and acid normality. In thecase of the liquor temperature deviating excessively from the set point,a threshold is set and the acid flow is subjected to a one-time acidflow correction that can bring the liquor temperature back to the setpoint quickly and prevent the liquor temperature from cycling.

[0102] The relationship of generator liquor composition and its boilingpoint may be expressed as:

T20=a(A+bC)+cP+d

[0103] Where T20 is the generator liquor temperature set point, A isacid normality, C is the chlorate molarity, P is the generator pressureand a, b, c and d are factors.

[0104] Based on the determined information relating to the parameters ofthe generator liquor, an acid normality predictor may be shown on theDCS at the plant for operator information. Without any laboratorychemical testing, the predictor advises the acid normality in thegenerator liquor based on the current temperature and chlorate molarity.

[0105] In this acid normality control strategy, acid flow correction asdescribed above effectively minimizes temperature swings. As the systemdetects an excessive liquor temperature deviation, the system initiatesa flow correction as the temperature moves back and close to the setpoint.

[0106]FIG. 10 shows the steps involved in chlorine dioxide productionrate feedback control. Information relating to the target chlorinedioxide production rate and methanol dilution target flow is receivedfrom the DCS while the calculated average chlorine dioxide productionrate is received as signal 7 (FIG. 8A) and the chlorine dioxideproduction rate deviation (DEV) is calculated. If the deviation is lessthan a predetermined percentage, the signal loops to the start of thecalculation, while, if the deviation is greater than the predeterminedpercentage, then along with the calculated new methanol dilution waterflow set point ((SP40), FIG. 6), the adjusted dilution water set point(SP4) is calculated from the deviation and the calculated water flow setpoint and the signal sent to the DCS.

[0107] A determination next is made whether the relationship$\frac{{SP4} - {SP40}}{SP4}$

[0108] is less than a predetermined percentage. If yes, the SP4=SP4 andthe corresponding signal is sent to the DCS. If no, then the change ofmethanol dilution water set point (SP4) is limited to a percentage ofthe initial flow set point (SP40) and the signal is sent to the DCS. Thesignal is also looped to the start of the calculations.

[0109] The conventional operation for chlorine dioxide production rateis merely feed-forward control on methanol flow that may or may not meetthe target production rate, since the methanol efficiency varies and isaffected by generator conditions. This invention uses a strategy to makeminor adjustments to the methanol flow as a function of actualproduction deviation from the target production rate. The methanol feedrate is allowed to fluctuate within a certain range driven by the offsetof actual production and the target. The methanol flow change is made insteps to minimize the possible impact on the chlorine dioxide strengthcontrol.

[0110]FIGS. 11A and 11B show the reboiler steam flow set pointdetermination. Information relating to target chlorine dioxideproduction rate, methanol flow, methanol consumption and acid density isreceived from the DCS while the calculated sodium chlorate feed solutionconcentration is received from the calculations of FIG. 8A (signal 8).The water load from chlorate, acid and methanol dilution water iscalculated, the water load from salt cake filter and acid sulfatemetathesis process (if present) is calculated, the water load generatedin the chemical reaction is calculated and information relating to theconstant water load, pump purges and seals, is added. The total waterload on the chlorine dioxide generator from all sources (SP50) iscalculated along with the minimum reboiler steam flow required toevaporate the water load (FMIN). If SP50<FMIN, then SP50 is FMIN and thesignal is sent to the DCS. If SP50 is not less than FMIN, then thereboiler steam flow set point deviation (DEV) is calculated. If DEV isgreater than a predetermined percentage, then SP50 is the value of SP50and the corresponding signal is sent to the DCS. In the event DEV is notgreater than the predetermined percentage, then the signal loops back tothe beginning of the calculations.

[0111] The conventional operation sets the steam flow in accordance tothe target production rate regardless of the varying operatingconditions. This invention monitors the ever-changing water input to thegenerator. The control strategy periodically quantifies all the watersources to maintain the mass balance of water load versus theevaporation rate at the reboiler and this determines the required steamflow rate.

[0112]FIGS. 12A and 12B show the steps required for generator levelcontrol. Information relating to generator level, methanol flow andmethanol consumption and make-up water valve position and its positionset point is received from the DCS and the projected chlorine dioxideproduction rate is calculated. The minimum (FMIN) and maximum (FMAX)reboiler steam flows for that projected production rate are calculated.A determination then is made as to whether the make-up water valve isopen. If no, then the steam bias required is calculated based ongenerator level deviation. If yes, a second determination is made as towhether the make-up water valve position is greater than its set point.If no, the signal loops back to the beginning of the calculations. Ifyes, then the steam bias required is calculated based on make-up watervalve position deviation. The reboiler steam set point (SP5) then iscalculated incorporating bias adjustment.

[0113] The change of bias (FIG. 12B) then is calculated and the averageof the change of bias (AVD) is calculated. A determination is made ifAVD is less than a predetermined percentage. If no, then the signalloops back to the commencement of the calculation. If yes, then theaverage of reboiler steam bias is calculated and the new steam set point(SP50) incorporating the average steam bias is calculated and forwardedto the DCS. The signal also is looped back to the beginning of thecalculation.

[0114] The reboiler steam set point (SP5) is compared to minimumreboiler steam flow from the projected production rate (FMIN). If SP5 isnot greater than FMIN, then SP5 is set to the value of FMIN and thesignal forwarded to the DCS and looped back to the start of thecalculation. If SP5 is greater than FMIN, then a further determinationis made as to whether SP5<FMAX. If no, then SP5 is set to the value ofFMAX and this value is forwarded to the DCS and looped back to the startof the calculation. If yes, then SP5 is set to the value of SP5 and thisvalue is forwarded to the DCS and looped back to the start of thecalculation.

[0115] Often the generator liquor level is not automatically controlledin conventional practice. In such case, the conventional operationinvolves setting a fixed steam flow according to the target productionrate and hardcopy feed table described above and manually orautomatically adjusting the make-up water to control the generatorliquor level. Such an operation is deficient inasmuch as it requiresunnecessarily high steam usage rates at all times in order to allow somecontinuous make-up water for control. In addition, the make-up waterwithout any applied constraint may significantly increase the water loadon the chlorine dioxide generator, resulting in more steam usage thatfurther contributes to elevated generator liquor carryover. Thisinvention uses a control strategy incorporating make-up water adjustmentas a fine level control in conjunction with steam flow adjustment as acoarse level control. The concept is to keep the make-up water valveposition always within a specified range. Step changes in reboiler steamflow are made according to the make-up water valve position or generatorliquor level deviation as described above with respect to FIGS. 12A and12B. Adjustment on both steam and make-up water flow not only assures aprecise generator liquor level control but also minimizes the make-upwater usage, resulting in steam savings.

[0116] There is a pre-set make-up water valve position beyond whichsteam is curtailed, thereby avoiding excessive steam usage owing toextra make-up water load. Proportional step changes in reboiler steamflow in comparison to the degree of generator level deviation prompts afast correction on the deviated generator liquor level withoutovershoot.

[0117] The combined reboiler steam and make-up water flow controlenables the generator liquor level to be maintained at its set point±1%. Not only can the steady level be maintained but disturbances can behandled. For example, in the event the generator liquor level surges dueto a water dump for some reason, the reboiler steam reacts immediatelyand the make-up water is fully shut off. Both actions rapidly decreasethe generator liquor level, while the reboiler steam is graduallyreduced as the generator liquor level moves towards its set point. Theupset is remedied in a short time and swings of generator level areavoided.

[0118] As seen in FIG. 3, the overall strategy includes a chlorinedioxide solution strength internal loop. This invention basically adoptsthe conventional control strategy with an additional enhancement. Therequired chilled water flow to the absorption tower is calculated fromthe target chlorine dioxide strength, production rate and all existingwater inputs to the tower. By accounting for all relevant operatingconditions, aqueous chlorine dioxide solution strength fluctuations areminimized.

[0119] In addition to these various controls, a maximum chlorine dioxidestrength and maximum temperature interlock set points are calculated toenable the operator to produce chlorine dioxide solution of the highestpossible strength while avoiding losses of chlorine dioxide from theproduct solution. The steps involved are shown in FIGS. 13A and 13B.Information relating to chlorine dioxide generator pressure, chlorinedioxide solution temperature, chlorine dioxide strength set point,process air implied valve position and maximum allowable chlorinedioxide solution strength by storage design (MS) is obtained from theDCS and it is then determined if the process air valve is open. In theevent the valve is open, the maximum chlorine dioxide solution hightemperature interlock set point (PTM) and the maximum chlorine dioxidesolution strength set point (SPM) take their predetermined values. Thesevalues are forwarded to the DCS and looped to the start of thecalculation.

[0120] If the process air valve is closed, the measured chlorine dioxidesolution temperature is converted to Kelvins, the water vapour above thesolution is calculated, the partial pressure of chlorine dioxide abovethe solution is calculated and corrected for air leakage, and Henry'sconstant is calculated for chlorine dioxide at the operating solutiontemperature. A new maximum chlorine dioxide solution strength (SPM) iscalculated at its partial pressure and temperature.

[0121] A determination then is made whether SPM<MS. If no, then SPM isthe value of MS and is forwarded to the DCS. If yes, then the newmaximum allowable chlorine dioxide high strength interlock set point iscalculated and entered. In this case, the value of SPM is SPM and thissignal is sent to the DCS.

[0122] In addition, Henry's constant is calculated at the given chlorinedioxide solution strength set point and chlorine dioxide partialpressure, the maximum allowable chlorine dioxide solution temperature atthe chlorine dioxide partial pressure is calculated, the chlorinedioxide solution temperature is converted from Kelvins and the newmaximum allowable chlorine dioxide solution high temperature interlockset point (PTM) is displayed and entered into the DCS and looped back tothe beginning of the calculation.

[0123] The conventional operation employs fixed interlock set points formaximum chlorine dioxide strength and solution temperature based onassumed constant operating conditions at the absorption tower base. Suchan operation is deficient in that the assumed conditions could bedifferent from the existing conditions so that either the productstrength is limited to be unnecessarily low or chlorine dioxide escapesfrom the absorption tower should the solution strength be higher thanthe existing operating conditions would allow. Consequently, this mayresult in a safety hazard, economic loss and/or a negative environmentalimpact. This invention calculates the maximum allowable chlorine dioxidesolution strength and temperature based on actual operating conditionswhich become the new interlock set points as the operating conditionschange. The solution strength can thus be set to optimum with the safetyassurance.

[0124] Maximum allowable chlorine dioxide solution strength, therefore,is determined mainly based on the solution temperature. The valueprovides the information to the operator so the product can be made toits highest possible strength without suffering from gas loss in theabsorption step. The advantages of providing an ongoing determination ofallowable chlorine dioxide solution strength include saving on chilledwater and subsequent chemical saving in the bleach plant. As thesolution temperature increases, the maximum allowable strengthdecreases, so the occurrence of unexpected chlorine dioxide gas slippagefrom absorption tower can be prevented.

[0125] The maximum allowable chlorine dioxide solution temperature,therefore, is determined based on the chlorine dioxide solutionstrength. Both maximum allowable strength and maximum allowabletemperature of the product are assigned as interlock set points and theybecome floating subject to the process conditions. Such a manner ofplant manipulation imparts the operator the best knowledge how toachieve the highest possible strength and best saving while assuring theprocess safety.

[0126] This invention enables chlorine dioxide plants worldwide to beeffectively controlled from a remote control location applying state ofthe art, industry accepted data communication technology.

[0127] The control system provided herein is a supervisory controlsoftware product that may be layered on existing chlorine dioxidegenerating plant DCS or incorporated into a new chlorine dioxidegenerating plant installation. Remote set points generated by thecomputer program running the control system on a suitable microprocessorin the manner described above are transferred to the ClO₂ plant DCScontrollers only when the plant operator permits the controllers toaccept the remote set points. The operator may disable supervisorycontrol by selecting the local control mode for any or all controllers.

[0128] The software may be customized and tuned for each customer andmay be stored on a dedicated server at a remote location. Selectedprocess data are extracted from the chlorine dioxide plant DCS atregular intervals using OPC (OLE for Process Control) server technologyand transferred to the control location using VPN (virtual privatenetwork) communication technology. The controller set points, determinedin the manner set forth in detail above, are transferred back to theClO₂ plant DCS in the same way. A schematic of the architecture is shownin FIG. 14.

[0129] In this manner, chlorine dioxide generating plants worldwide canbe controlled effectively from a remote location. Such a remotemonitoring system makes professional maintenance always available tocustomers, thereby further improving operation reliability.

[0130] As described above and as seen from the drawings, the entirechlorine dioxide generating plant can be manipulated and controlledbased on the target chlorine dioxide production rate and by usingcomputer-aided calculations to maintain the target production rate,operation stability and optimum efficiency. Apart from initial testingto establish the plant specific chemical consumption, only occasionallaboratory testing of generator liquor sodium chlorate concentration andsolids content is required to ensure compliance with targets. The use ofthe control strategies described herein enables optimum use of feedchemicals, particularly of the most expensive chemical, sodium chlorate,to be employed for a given chlorine dioxide target production rate. Thisresult has been demonstrated in an experimental commercial scaleoperation, where an overall savings of greater than 2% of chlorate feedwas achieved for a target production rate of over 30 tons of chlorinedioxide per day.

[0131] It is also possible to employ a combination of the variousadvanced control strategies described herein with the conventionalstrategies, if desired, but these options are considered less desirablethan using all the advanced control strategies.

[0132] In addition, the above specific description assumes that methanolis used as the reducing agent in the chlorine dioxide generatingprocess. However, corresponding strategies may be used when otherreducing agents, such as hydrogen peroxide, are employed, such as inR11® and SVP-HP® processes. Similar strategies may also be applicable tothe non-SVP processes for chlorine dioxide generation, such as R2,Mathieson, Solvay, HP-A and others.

[0133] Other possible improvements involve linking the chlorineproduction rate to the overall bleach plant demand for chlorine dioxide,preferably in conjunction with maintaining a steady chlorine dioxidestorage level. In such a case, a manual input of the target productionrate is no longer required.

[0134] Another potential improvement would be the generator solidspercent control based on controlling generator liquor density andsaltcake filter operation parameters.

[0135] Yet another possible improvement could be the implementation ofan on-line chlorate molarity analyser of the type described, for examplein the U.S. Pat. No. 5,948,236, assigned to the assignee hereof.

EXAMPLE

[0136] This Example illustrates the application of the present inventionto chlorine dioxide generation plant.

[0137] A commercial chlorine dioxide generating plant according to FIG.1 was operated both conventionally and by utilizing the control systemdescribed herein and shown in FIGS. 2 to 13B.

[0138] In this plant scale study over a period of 12 months, the controlsystem described herein in comparison to conventional operation resultedin an increase in yield of chlorine dioxide based on chlorate of over 2%and a significantly-decreased requirement for lab testing.

[0139] In addition, reductions were achieved in the variability ofcertain chlorine dioxide generator process parameters, as set forth inthe following Table: TABLE % Variability Parameter Reduction Deviationfrom Chlorine Dioxide Production Target 82% Deviation from ChlorineDioxide Strength Target 37% Chlorine Dioxide Solution Strength to BleachPlant 35% Generator Level 35% Acid Normality in Generator Liquor  8%Chlorate Molarity in Generator Liquor 18%

SUMMARY OF DISCLOSURE

[0140] In summary of this disclosure, the present invention enablesimproved control of the operation of a chlorine dioxide generatingprocess to be achieved based on a target chlorine dioxide productionrate by employing a series of advanced control strategies which providefor optimum chemical usage. Modifications are possible within the scopeof the invention.

What we claim is:
 1. A continuous process for the generation of chlorine dioxide at a target production rate, which comprises: reducing chlorate ions in an aqueous acid reaction medium in a reaction zone using a reducing agent and sulfuric acid at the boiling point of the reaction medium under a subatmospheric pressure, removing a gaseous admixture comprising water vapour and chlorine dioxide from the reaction medium, absorbing said gaseous admixture in chilled water in an absorption zone to provide a product aqueous solution of chlorine dioxide, removing a slurry of spent reaction medium and by-product crystalline sulfate from the reaction zone, separating the crystalline sulfate as a by-product from spent reaction medium, adding make-up quantities of chlorate ions, reducing agent and sulfuric acid to the spent reaction medium to form a make-up feed, evaporating water inputted to the process from all sources using steam fed to a reboiler, recycling the make-up feed to the reaction zone, and computer controlling said process on the basis of a desired target chlorine dioxide production rate as the sole input from an operator to a computer program effecting such computer control.
 2. The process of claim 1 wherein said chlorate ions are provided by sodium chlorate.
 3. The process of claim 2 wherein said computer controlling operation comprises: continuously monitoring the target production rate of aqueous chlorine dioxide solution for changes therein, continuously monitoring the flow rates of sodium chlorate, reducing agent, sulfuric acid, reboiler steam and chilled water to the process, and modifying the initial set points of all said flows in accordance with the changed target production rate.
 4. The process of claim 3 wherein said computer controlling operation comprises: continuously monitoring the production rate of aqueous chlorine dioxide solution for deviations from the target production rate, and modifying the reducing agent flow rate to maintain the production rate at its target.
 5. The process of claim 1 wherein the maximum allowable chlorine dioxide product solution strength and maximum allowable temperature are advised.
 6. The process of claim 4 including: continuously monitoring the specification of all material feeds, and modifying the appropriate flow rate set points of said feeds to the reaction zone based on the target production rate and in response to changes in material specification.
 7. The process of claim 4 including: continuously monitoring sodium chlorate solution physical properties, temperature and density, and, on this basis, creating an on-line virtual chlorate solution analyzer that determines the volumetric concentration of the sodium chlorate solution.
 8. The process of claim 7 wherein said on-line virtual chlorate solution analyzer provides an accuracy of about ±0.3% in the sodium chlorate concentration range of about 450 to about 750 gpL.
 9. The process of claim 4 including: continuously monitoring the mass input of sodium chlorate to the reaction medium, continuously monitoring the mass consumption of sodium chlorate by the process, and modifying the flow of sodium chlorate to the reaction medium to correspond to the mass consumption of sodium chlorate so as to maintain the sodium chlorate concentration in the reaction medium substantially constant.
 10. The process of claim 4 including: establishing the boiling temperature set point of the reaction medium based on the expected reaction medium composition, continuously monitoring the temperature of the aqueous acid reaction medium, continuously controlling the temperature of the reaction medium in order to maintain a constant acid normality in the reaction medium, and continuously predicting the acid normality of the aqueous acid reaction medium from the temperature and the chlorate molarity of the aqueous solution.
 11. The process of claim 10, including: continuously determining whether the temperature of the aqueous reaction medium differs from the temperature set point, and correcting such deviation by suitable modification to the acid flow rate to the aqueous reaction medium.
 12. The process of claim 4 including: continuously controlling sodium chlorate molarity in the aqueous reaction medium on the basis of continuously determined system mass balance and adaptive yield tracking.
 13. The process of claim 4 including: periodically laboratory testing the concentration of sodium chlorate in the reaction medium and monitoring the results of such laboratory testing for a trend in alteration of the concentration of the sodium chlorate in the reaction medium, determining whether or not the concentration of sodium chlorate in the reaction medium has changed in the same direction in a predetermined number of said periodic laboratory tests, in the event, such a change has taken place and provided that the operator has selected the “ADAPTIVE YIELD” function switch, initiating a yield calculation using a series of laboratory tests to determine the applicable adaptive yield.
 14. The process of claim 4 including: periodically laboratory testing the concentration of sodium chlorate in the reaction medium, determining whether or not the concentration of sodium chlorate in the reaction medium has changed from a target value, and in the event such a change has taken place and provided the operator has selected the “LAB TEST” function switch, applying a one-time bias to the flow rate of sodium chlorate to the reaction medium for a predetermined time to adjust the sodium chlorate concentration in the reaction medium to the target value.
 15. The process of claim 4 including: maintaining the level of reaction medium in the reaction zone substantially constant by continuously balancing the volume of water flowing to the process and the volume of water evaporated from the reaction medium.
 16. The process of claim 4 including continuously determining and displaying the acid normality of the reaction medium.
 17. The process of claim 4 including continuously determining and displaying the concentration of sodium chlorate in the reaction medium.
 18. The process of claim 2 wherein said reducing agent is methanol.
 19. The process of claim 18 including: continuously monitoring the production rate of aqueous chlorine dioxide solution, and modifying the feed rate of methanol to the reaction medium in response to fluctuations within a predetermined range based on the initial methanol flow set point.
 20. A continuous process for the generation of chlorine dioxide at a predetermined production rate, which comprises: reducing sodium chlorate in an aqueous reaction medium in a reaction zone using methanol and sulfuric acid at the boiling point of the reaction medium under a subatmospheric pressure, removing a gaseous admixture comprising water vapour and chlorine dioxide from the reaction medium, absorbing said gaseous admixture in chilled water in absorption zone to provide a product aqueous solution of chlorine dioxide, removing a slurry of spent reaction medium and by-product crystalline sodium sulfate from the reaction zone, separating the crystalline sodium sulfate as a by-product from spent reaction medium, adding make-up quantities of sodium chlorate, methanol and sulfuric acid to the spent reaction medium to form a make-up feed, evaporating water inputted to the process from all sources using steam fed to a reboiler, recycling the make-up feed to the reaction zone, and computer controlling said process to produce chlorine dioxide from the reactants with optimum chemical usage on the basis of a desired chlorine dioxide production rate as the sole input to a computer program effecting such computer control.
 21. The process of claim 20 wherein said computer program monitors parameters of the process, including: aqueous chlorine dioxide solution production rate pressure of the reaction zone temperature, liquid level and sodium chlorate concentration of the reaction medium flow rate of chilled water to the chlorine dioxide absorption step flow rate of aqueous sodium chlorate solution, sulfuric acid and aqueous methanol to the reaction medium flow rate of steam to the reboiler flow rate of make-up water to the process density and temperature of aqueous sodium chlorate feed density of aqueous methanol feed said computer program further generating modification to flow controllers control of the flow rate of chilled water to the chlorine dioxide absorption step aqueous sodium chlorate, sulfuric acid and aqueous methanol to the reaction medium steam to the reboiler
 22. The process of claim 21 wherein said computer program continuously monitors production rate of aqueous chlorine dioxide solution and compares the monitored production rate to the target production rate until a deviation resulting from fluctuations in the process is detected, whereupon the computer program initiates changes in the flow rate of methanol to restore the production rate to the target value.
 23. The process of claim 22 wherein acid flow set point is determined by: determining current sulfuric acid consumption, sulfuric acid concentration and target chlorine dioxide production rate calculating the new acid flow set point
 24. The process of claim 22 or 23 wherein methanol dilution water set point is determined by: determining current methanol consumption, methanol density and target chlorine dioxide production rate calculating the new methanol flow set point calculating the new methanol dilution water flow set point (SP40).
 25. The process of claim 22, 23 or 24 wherein chilled water to chlorine dioxide absorption step set point is determined by: determining current chlorine dioxide solution strength set point, reboiler steam flow, chilled water flow to chlorine dioxide storage tank vent scrubber and target chlorine dioxide production rate calculating the total required water flow at the target chlorine dioxide production rate calculating condensate flow from an indirect contact cooler for said gaseous admixture to the absorption tower calculating a new chilled water to absorption tower flow set point (SP70) calculating the minimum required chilled water flow to the absorption tower (SPMN) determining if SP70>SPMN if SP70 does not exceed SPMN, then the new chilled water flow set point (SP70) is SPMN if SP70 exceeds SPMN, then the new chilled water flow set point (SP70) is SP70.
 26. The process of claim 25 including: determining if the chlorine dioxide strength or scrubber flow set points have changed if not, effecting chlorine dioxide solution strength control.
 27. The process of any one of claims 22 to 26 wherein aqueous sodium chlorate solution feed control to the reaction medium is determined by: determining current sodium chlorate solution flow, density and temperature, chlorine dioxide flow rate to storage and chlorine dioxide solution strength calculating average chlorine dioxide production rate calculating sodium chlorate usage based on actual chlorine dioxide production rate and yield calculating sodium chlorate concentration in sodium chlorate feed solution calculating required sodium chlorate solution flow rate determining current sodium chlorate molarity and percent solids target, reaction medium level and laboratory test data with respect to sodium chlorate molarity and percent solids in the reaction medium determining if the laboratory test data has changed in the event, that the laboratory test data has changed, determining whether an adaptive yield mode or lab test mode is selected (A)—in the event the lab test mode is selected, calculating the reaction medium volume at the actual operating conditions, calculating the aqueous sodium chlorate mass inventory in the reaction medium at actual operating conditions, calculating reaction medium reference liquid volume at reference conditions calculating reference aqueous sodium chlorate mass inventory at reference conditions calculating the difference between said sodium chlorate mass inventories calculating an aqueous sodium chlorate solution flow adjustment bias and applying the bias to the calculated aqueous sodium chlorate flow rate for a predetermined time calculating the aqueous sodium chlorate solution flow set point incorporating the bias for the predetermined time (B)—in the event the adaptive yield mode is selected and in the event, the number of laboratory tests criterion has been met, calculating the chlorine dioxide mass output beginning from the first valid laboratory test of said number of laboratory tests calculating the total sodium chlorate mass input beginning from the first valid laboratory test, calculating a corrected sodium chlorate consumption using the last valid laboratory test of said number of laboratory tests in relation to the predicted sodium chlorate concentration in the reaction medium calculating the sodium chlorate adaptive yield correction factor calculating the aqueous sodium chlorate flow set point incorporating the adaptive yield correction factor.
 28. The process of any one of claim 22 to 27 wherein aqueous reaction medium acidity control is effected by: determining the current sulfuric acid flow, aqueous reaction medium temperature, reaction medium target acid normality and sodium chlorate molarity and reaction zone pressure calculating from the latter information, the new sulfuric acid flow set point (SP20) and said flow controller set point limits calculating the reaction medium temperature controller set point (T20) calculating the deviation of reaction medium temperature from set point in the event the deviation exceeds a predetermined value, monitoring the deviation and, at the time the reaction medium temperature approaches the set point, resetting the current acid flow set point to the previously calculated value (SP20)
 29. The process of any one of claims 22 to 28 wherein chlorine dioxide production rate feedback control is effected by: determining the deviation (DEV) of chlorine dioxide production rate from the target in the event the deviation exceeds a predetermined value, calculating an adjusted methanol dilution water set point (SP4) from the deviation (DEV) and the initial methanol dilution set point (SP40) determining if $\frac{{SP4} - {SP40}}{SP4}$

is less than a predetermined value, in which case the adjusted methanol dilution water set point (SP4) is employed, if not, limiting the methanol dilution water set point (SP4) increase to a predetermined percentage of the initial methanol dilution water flow set point (SP40).
 30. The process of any one of claims 22 to 29 wherein reboiler steam flow set point determination is effected by: determining the target chlorine dioxide production, methanol flow rate, methanol consumption and sulfuric acid density calculating the water load from sodium chlorate, sulfuric acid and methanol dilution water calculating water load from a salt cake filter used to effect separation of crystalline sodium sulfate from spent reaction medium calculating the water load generated in the chemical reaction adding water load from pump purges and seals calculating the reboiler steam flow set point for the total water load from all sources (SP50) calculating the minimum reboiler steam flow (FMIN) determining if SP50<FMIN, in which case SP50=FMIN in the event SP50 is not less than FMIN, calculating the reboiler steam flow set point deviation and if the deviation exceeds a predetermined value, the SP50=SP50.
 31. The process of any one of claims 22 to 30 wherein the reaction medium liquid level control is effected by: determining the current reaction medium liquid level, methanol flow rate, methanol consumption, make-up water valve position and position set point calculating the projected chlorine dioxide production rate calculating the minimum (FMIN) and maximum (FMAX) reboiler steam flows determining whether the make-up water valve is open in the event the valve is closed, calculating a steam bias based on reaction medium liquid level deviation in the event the valve is open, determining if the make-up water valve position is greater than the predetermined make-up water valve position set point and, if so, calculating a steam bias based on make-up water valve position deviation calculating the reboiler steam set point incorporating the bias adjustment (SP5) calculating change in bias calculating average of change of bias (AVD) determining if AVD is less than a predetermined value and, if so, calculating the average of reboiler steam flow bias calculating the new steam set point (SP50) incorporating the average steam flow bias determining if SP5>FMIN and, if not, then SP5=FMIN if SP5>FMIN, determining if SP5<FMAX, if so, then SP5=SP5 and, if not, SP5=FMAX
 32. The process of any one of claims 22 to 31, wherein the maximum allowable chlorine dioxide solution strength and temperature interlock is determined by: determining the current reaction zone pressure, chlorine dioxide solution temperature, chlorine dioxide solution strength set point, process air implied valve position and maximum allowable chlorine dioxide solution strength by storage design (MS) determining if the process air valve is open and if so, then the maximum chlorine dioxide solution strength and temperature interlock set points take their predetermined values in the event the process air valve is close, converting the chlorine dioxide solution temperature to Kelvins calculating the water vapour pressure above the chlorine dioxide solution calculating the chlorine dioxide partial pressure above the solution and correcting for air leakage calculating Henry's constant for chlorine dioxide at the operating solution temperature calculating the new maximum chlorine dioxide solution strength (SPM) at its partial pressure and temperature determining if SPM<MS and if not SPM=MS and if so, displaying and entering the new maximum allowable chlorine dioxide solution strength interlock set point and SPM=SPM calculating Henry's constant for the chlorine dioxide solution from its strength set point and chlorine dioxide partial pressure converting the chlorine dioxide solution temperature from Kelvins displaying and entering the maximum allowable solution temperature interlock set point (PTM). 